Generally, a conventional optical fiber coupler has a drawback that it easily changes in characteristics and is damaged due to external force or temperature change, because it has a small outer diameter portion of a few tens of .mu.m. Therefore, the conventional optical fiber coupler is designed such that the coupler for the melted, coupled and elongated optical fibers is fixed to a reinforcement substrate having a linear expansion coefficient as much as quartz, so that the characteristics are stable.
As the reinforcement substrate, a plate member is used as described in Japanese Utility Model Application Laying-open No. 23408/1989 and a pipe member is used as described in Japanese Patent Application Laying-open No. 63907/1989, for example.
On the other hand, thermosetting adhesive or ultraviolet cure adhesive of epoxy, urethane acrylate, and cyanoacrylate is used as the adhesive for adhering the conventional coupler.
Recently, a high density optical communication line has developed so that the number of optical couplers increases. Therefore, since the conventional optical fiber coupler is designed to couple a pair of optical fibers to each other, there are caused problems that an occupied area by the optical fiber coupler increases and accommodation of excess portions at the ends of the optical fibers is complex.
On the contrary, as disclosed in Japanese Patent Application Laying-open No. 295211/1990, for instance, the reinforcing method of the optical fiber coupler is proposed in which after an optical fiber coupler for coupling two optical fibers is fixed to a reinforcement member so that a plurality of reinforcement members are accommodated in a package, the optical fibers are coupled outside of the package by a tape to construct an arrayed optical fiber coupler. Another method of reinforcing an optical fiber coupler is proposed in which optical fiber couplers for each coupling two optical fibers to each other are arranged on a fixing member having a plurality of grooves in a comb manner, as disclosed in Japanese Patent Application Laying-open No. 254406/1988.
The methods disclosed in the above Japanese Patent Application Laying-open No. 295211/1990 and Japanese Patent Application Laying-open No. 254406/1988 are complicated and have a problem that it takes a long time to form an arrayed optical fiber coupler because the optical fiber couplers for each coupling two optical fibers are mounted in a high density manner.
For this reason, as disclosed in Japanese Patent Application Laying-open No. 120510/1990, the method is tried in which an arrayed optical fiber coupler is directly constituted from arrayed optical fiber ribbons. In a case that the arrayed optical fiber coupler is fixed to a reinforcement substrate, however, there is a problem that the transmission characteristics are remarkably degraded due to temperature change or humidity change.
FIGS. 10A, 10B and 10C show a reinforcement structure in a conventional arrayed optical fiber coupler. FIG. 10A is a plan view of the reinforcement structure, FIG. 10B is a perspective view of a reinforcement case used in the reinforcement structure shown in FIG. 10A, and FIG. 10C is a cross sectional view of an arrayed optical fiber ribbon used in the reinforcement structure shown in FIG. 10A. A reference numeral 1 denotes an arrayed optical fiber ribbon in FIG. 10C. This arrayed optical fiber ribbon 1 is constituted by collectively coating four optical fibers 2 each having a protection coating layer 2a with a coating layer 1a. A reference numeral 3 denotes a reinforcement case in FIGS. 10A and 10B. 0n the upper surface 3a of the reinforcement case 3, a pair of fixed walls 3b extending along a longitudinal direction of the case in parallel to each other at both side edges are provided to prevent the displacement of the optical fiber ribbon 1 by the reinforcement structure.
In this arrayed optical fiber ribbon 1, after part of the coating layer 1a and protection coating layer 2a of each optical fiber is removed to expose a glass portion of the optical fiber 2, the ribbons 1 are overlain in a vertical direction and corresponding glass portions 2b are heated and elongated to be melted and coupled. The glass portion 2b is fixed to the upper surface 3a and the fixed wall 3b of the reinforcement case 3 by an adhesive layer 4, as well as the coating resin layer 1a of the optical fiber ribbon 1.
However, because distortion remained in the collectively coating resin layer 1a from when the arrayed optical fiber ribbon 1 has been manufactured is released in a form of contraction of the collectively coating resin layer 1a due to temperature change or humidity change, a relative shift is caused between the collectively coating resin layer 1a and the glass portion 2b including a melted, bonded and elongated portion. Thus, in the conventional arrayed optical fiber coupler, because the collectively coating resin layer 1a is strongly fixed to the reinforcement case 3, a stress is generated at a melted, bonded and elongated portion having a small diameter so that the transmission characteristics of the optical fiber coupler is changed.
In a case where four or more optical fibers are collectively adhered and fixed to the reinforcement member, the optical fiber coupler does often not have sufficient circumstance resistance characteristics. For instance, an allowable change value is generally 0.2 dB or less in a heat cycle test in which temperature load of -20.degree. to +60.degree. C. is applied to the optical fiber coupler to examine characteristics change. However, in the above structure, the change value of about 0.5 dB is often observed, as shown in FIG. 11. The change value needs to be 0.2 dB or less after 100 hours in a characteristics change test under 60.degree. C. and 95%. In the above structure, the more than 0.3 dB change values are frequently observed, as shown in FIG. 12. It should be noted that in FIG. 11 the abscissa represents time length of the heat cycle test, the ordinate represents temperature in the heat cycle test and change value of coupling loss due to the heat cycle test and that in FIG. 12 the abscissa represents time length during which a sample is exposed under humidity and heat condition and the ordinate represents change value of coupling loss under the humidity and heat condition.
An object of the present invention is to provide an arrayed optical fiber coupler and a method of manufacturing the same in which the arrayed optical fiber coupler has a reinforcement structure with sufficient circumstance resistance characteristics and, even if a relative shift is caused between a collectively coating resin layer and a glass portion of optical fiber ribbon because of distortion remained in the collectively coating resin layer from when the optical fiber ribbon is manufactured, a melted, coupled and elongated portion is not influenced due to the relative shift.